Molybdenum disulphide (MoS2) belongs to the family of single-layer transition metal dichalcogenides (TMDs). These are quasi-two-dimensional materials with the chemical formula MX2, consisting of an atomic plane of a transition metal M (Ti, Nb, Mo, Re) sandwiched between the atomic planes of a chalcogen X (S, Se or Te). These materials go from being indirect bandgap semiconductors in the bulk to direct bandgap semiconductors when scaled down to monolayer thickness. These monolayers efficiently absorb and emit light and so might find use in a variety of electronics and optoelectronics device applications.

However, the problem is that the ON-current in transistors made from 2D semiconductors thus far has been considerably lower than that of silicon devices. This is because it is difficult to reduce both the contact resistance and channel length in devices made from atomically thin materials.

Synthetic nanowire as the lift-off mask

Now, researchers led by Xiangfeng Duan say that they have overcome this challenge in their new approach, which involves using a synthetic nanowire as the lift-off mask to define an ultra-short channel device with pristine MoS2 channels and self-aligned low-resistance graphene/metal hybrid contacts. The sub-100 nm transistor produced has a record high ON-current of 0.83 mA/µm at 300K and 1.48 mA/µm at 20K, which compares well to that of devices based on silicon.

“Specifically there are three steps to our fabrication method,” explains team member Yuan Liu. “First, we exfoliate few-layer MoS2 onto a SiNx substrate with SiNx as the back-gate dielectric. Next, we mechanically align the pre-synthesized silicon nanowires on top of the MoS2 flake. Then, we transfer a single layer of graphene (grown by chemical vapour deposition, or CVD) on top of the nanowire/MoS2 heterostructure using a standard wet transfer technique and deposit nickel/gold as the hybrid metal contacts. Finally, we mechanically remove the nanowire and the graphene/metal stack using sticky tape, leaving behind two isolated graphene/metal hybrid pads as the source and drain electrodes.”

"Viable, alternative electronic materials"

With their record high current density and performance comparable to that of state-of-the-art silicon transistors, we believe that 2D semiconductors could be viable, alternative electronic materials for the post-silicon era, he tells nanotechweb.org. We hope our results will stimulate further interest from both academia and industry for this new class of materials and push their potential further.

The team say that it is now busy trying to minimize “parasitic” capacitance in the device to create high-speed devices. “Our studies also clearly indicate that contact resistance and phonon scattering (vibrations of the crystal lattice) are the main limiting factors for 2D semiconducting transistors, so we need to reduce these even further to push performance ever higher,” says Duan.

The researchers report their record-breaking devices in Nano Letters DOI: 10.1021/acs.nanolett.6b02713.